1. Field of the Invention
The present invention relates, in general, to a two-stage centrifugal compressor used in refrigeration cycles and, more particularly, to an improvement in such a compressor to allow the electric motor to be cooled by refrigerant of a refrigeration cycle without having another external motor cooler, thus simplifying the refrigerant line of the refrigeration cycle thereby resulting in an improvement in both the operational performance and operational efficiency of the cycle.
2. Background Art
As well known to those skilled in the art, two-stage centrifugal compressors have been typically used in large-scaled refrigeration systems. However, it is necessary in recent days to actively study such a type of compressors to reduce the size of them since it is desired to use them with indoor air conditioners for houses or automobiles.
Conventional two-stage centrifugal compressors for refrigeration systems are composed of an electric motor set at the central portion within a motor housing, with the motor consisting of a rotor and a stator. An impeller having a plurality of blades is set on both ends of a rotating shaft of said rotor. First and second compression parts are provided to the opposite ends of the motor housing. In an operation of a refrigeration system, refrigerant circulates through a main refrigerant line of the system and is compressed at the first and the second compression parts by said impellers of the compressor. The compressed refrigerant from the compression parts passes through the main refrigerant line while absorbing heat from the electric motor and cooling the motor.
In the operation of the refrigeration system, the two-stage centrifugal compressor receives refrigerant from an evaporator and primarily compresses the refrigerant at the first compression part, and secondarily compresses the primarily compressed refrigerant at the second compression part, and so the refrigerant becomes highly heated and highly pressurized gas refrigerant. The gas refrigerant having high temperature and high pressure is fed from the compressor to a condenser. In order to suck refrigerant from the evaporator and to compress the refrigerant, and to circulate the pressurized refrigerant in the refrigeration system during a refrigeration cycle, it is necessary to rotate the electric motor at a high speed. Due to such a high speed rotating motion of the motor, much heat is generated from both the motor and its bearing parts and deteriorates the operational performance of the compressor in addition to a reduction in operational reliability of the compressor.
Therefore, it is necessary to appropriately and effectively cool the motor and the bearing parts. In an effort to accomplish the above object, the construction of both a motor cooling system and a refrigerant line of a two-stage centrifugal compressor has been actively studied. As a result of such active studies, several techniques for cooling the motor and the bearing parts of such a two-stage centrifugal compressor have been proposed as follows.
In a conventional two-stage centrifugal compressor of FIG. 1a, gas refrigerant is sucked from an evaporator into the compressor C through a first refrigerant line L1. In such a case, the refrigerant is primarily compressed at the first compression part C1. The refrigerant is, thereafter, fed into the second compression part C2 of the compressor C through an external refrigerant line L2 so as to be secondarily compressed at that compression part C2, and so the refrigerant becomes highly heated and highly pressurized gas refrigerant. The gas refrigerant, having a high temperature and a high pressure, is fed from the compressor C into a condenser. In the above compressor C, a motor cooling cycle is separately provided in the fixed jacket Mj of the electric motor M. In the motor cooling cycle provided in the jacket Mj, refrigerant from the compressor C circulates through another refrigerant line L3 under the control of a separate cooling unit Cu, thus cooling the motor M.
FIG. 1b shows a refrigerant cycle diagram of a conventional two-stage centrifugal compressor according to another embodiment of the prior art. In this compressor C, gas refrigerant is sucked from an evaporator into the compressor C through a first refrigerant line L1. The refrigerant is thus primarily compressed at the first compression part C1 of the compressor C and is secondarily compressed at the second compression part C2 prior to being fed into a condenser in the same manner as that described for the embodiment of FIG. 1a. The condenser condenses the refrigerant so as to convert the phase of the refrigerant from a gas phase into a liquid phase prior to feeding the refrigerant to an evaporator. In this compressor C, a part of the condensed refrigerant is sucked into and flows in the fixed jacket Mj of the electric motor M. The liquid refrigerant is evaporated while flowing in the jacket Mj and is joined to an external refrigerant line L4 prior to being sucked into the second compression part C2 of the compressor C. At the second compression part C2 of the compressor C, the gas refrigerant from the jacket Mj is compressed to become a highly pressurized refrigerant.
FIG. 1c shows a refrigerant cycle diagram of a conventional two-stage centrifugal compressor according to a further embodiment of the prior art. In this compressor C, gas refrigerant is sucked from an evaporator into the compressor C through a first refrigerant line L1 and flows in the interior of the compressor C while cooling the electric motor M. The refrigerant is, thereafter, sucked into the first compression part C1 of the compressor C through an external refrigerant line L2, thus being primarily compressed at that compression part C1 The refrigerant is sucked from the first compression part C1 into the second compression part C2 of the compressor C through the external refrigerant line L2, thus being secondarily compressed at that compression part C2 prior to being fed into a condenser.
FIG. 1d shows a refrigerant cycle diagram of a conventional two-stage centrifugal compressor according to still another embodiment of the prior art. In the embodiment of FIG. 1d, gas refrigerant is sucked from an evaporator into the compressor C through a first refrigerant line L1, and is primarily compressed at the first compression part C1 of the compressor C. The refrigerant is, thereafter, fed into the second compression part C2 of the compressor C through an external refrigerant line L2 so as to be secondarily compressed at that compression part C2, and so the refrigerant becomes highly heated and highly pressurized gas refrigerant. The gas refrigerant, having a high temperature and a high pressure, is fed from the compressor C into a condenser. In the above compressor C, another refrigerant line L5 extends through the interior of the compressor C prior to being joined to the external refrigerant line L2. Gas refrigerant, which is separated from liquid refrigerant at a vapor liquid separator of a refrigeration cycle, flows into the compressor C through the refrigerant line L5. On the other hand, an additional refrigerant line L4 extends through the fixed jacket Mj of the electric motor M prior to being joined to the external refrigerant line L2. The condensed liquid refrigerant from the condenser flows through the refrigerant line L4.
FIG. 1e shows a refrigerant cycle diagram of a conventional two-stage centrifugal compressor according to still another embodiment of the prior art. In the embodiment of FIG. 1e, gas refrigerant is sucked from an evaporator into the first compression part C1 of the compressor C through a first refrigerant line L1, and is primarily compressed at that compression part C1. The refrigerant, thereafter, flows through an external refrigerant line L2 so as to pass through the interior of the compressor C while cooling the electric motor. The refrigerant is, thereafter, fed into the second compression part C2 of the compressor C through the external refrigerant line L2, thus being secondarily compressed at that compression part C2. In this compressor C, gas refrigerant, which is separated from liquid refrigerant at a vapor liquid separator of a refrigeration cycle, flows into the second compression part C2 of the compressor C through an additional refrigerant line L5, thus being secondarily compressed at that compression part C2. The secondarily compressed refrigerant is discharged from the compressor C into a condenser of the refrigeration cycle.
FIG. 1f shows a refrigerant cycle diagram of a conventional two-stage centrifugal compressor according to still another embodiment of the prior art. In the embodiment of FIG. 1f, gas refrigerant is sucked from an evaporator into the first compression part C1 of the compressor C through a first refrigerant line L1, and is primarily compressed at that compression part C1. The refrigerant, thereafter, flows into the second compression part C2 of the compressor C through both the interior of the compressor C and an external refrigerant line L2 thus being secondarily compressed at that compression part C2. In this compressor C, gas refrigerant, which is separated from liquid refrigerant at a vapor liquid separator of a refrigeration cycle, flows into the second compression part C, of the compressor C through another refrigerant line L5, thus being secondarily compressed at that compression part C2. The secondarily compressed refrigerant is, thereafter, discharged from the compressor C into a condenser of the refrigeration cycle.
However, the above-mentioned conventional two-stage centrifugal compressors for refrigeration system have disadvantages as follows. That is, such a compressor C necessarily has a complicated refrigerant passage structure, which allows gas refrigerant to be primarily compressed in the interior of the compressor or at the first compression part C1 of the compressor C and to be fed into the second compression part C2 of the compressor C through an external refrigerant line L2 so as to be secondarily compressed at that compression part C2. Such a refrigerant passage structure forces the compressor C to have additional refrigerant lines L3-L5 in addition to the main refrigerant line L1, which is necessarily provided at the compressor C for sucking, compressing and discharging the refrigerant. Due to such additional refrigerant lines L3-L5, the refrigerant passage structure of such conventional two-stage centrifugal compressors is undesirably complicated since it consists of a plurality of refrigerant lines L1-L5. Such a complicated refrigerant passage structure also disturbs flowing of refrigerant, thus causing a refrigerant pressure loss. As a result, this finally deteriorates the operational performance of such compressors in addition to a reduction in both work efficiency and productivity while producing such compressors.
Another problem experienced in the conventional two-stage centrifugal compressors resides in that the compressors necessarily have a separate motor cooling unit Cu, having its own refrigerant flowing cycle separate from a main refrigeration cycle, so as to cool both the electric motor and mechanical frictional parts of the compressors. This finally increases the production cost of the two-stage centrifugal compressors for refrigeration systems.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a two-stage centrifugal compressor, which is designed to primarily compress gas refrigerant at a first compression chamber, or the first compression part, of the compressor and to allow the primarily compressed refrigerant to pass through the interior space of an external housing so as to reach a second compression chamber provided at the upper end of the external housing, and to secondarily compress the refrigerant at the second compression chamber, and which thus has a simplified refrigerant passage structure and effectively cools the electric motor and the bearing parts for the refrigeration cycle without having another external cooling unit and reduces a consumption of electric power, and improves the operational performance and operational efficiency of a refrigeration system.
In order to accomplish the above object, the present invention provides a two-stage centrifugal compressor, comprising an electric motor positioned at the central portion within an external housing, the motor being composed of a fixed Jacket including a stator provided with a coil, and a rotor having a rotor core provided on a rotating shaft at a position opposite to the stator, and rotatably supported by radial gas bearings and thrust gas bearings at upper and lower ends of the rotating shaft, and first and second compression parts provided at the upper and lower ends of the external housing of the motor and respectively having an impeller, further comprising: a plurality of internal fluid passages regularly formed on the internal surface of each of the upper and lower end portions of the external housing so as to allow the first and second compression parts to communicate with each other through the interior space of the external housing; and upper and lower bearing housings fixedly mounted to the upper and lower ends of the external housing, with a plurality of fluid openings being regularly formed on the upper and lower bearing housings and communicating with both compression chambers of the first and second compression parts, corresponding to the internal fluid passages of the external housing. In the two-stage centrifugal compressor, the second compression part has the second compression chamber within a compressing casing, receiving an inlet part of a diffuser and surrounding outer part thereof, provided with a refrigerant discharging port, and has an internal cover with a fluid passage communicating with the fluid openings of the upper and lower bearing housings.